The present invention relates to a toner for use in an electrophotography, and more particularly to a toner for use in an electrophotography, which has high bonding strength to an ink-repellent surface such as a dry planographic printing plate in particular and can improve the printing durability of the printing plate.
The dry planographic printing plate for use in an electrophotographic system comprises a substrate such as paper and an ink-receptive toner deposited imagewise thereon by an electrophotographic process the surface of said substrate being coated with an ink-repellent material such as silicone. In printing with such a printing plate, inking is directly made all over the surface of the printing plate without using any fountain solutions, whereby ink does not adhere to the silicone-coated surface, but adheres only to the imagewise toner, so that the ink image is transferred to paper and printing is effected.
Thus, the printing with the dry planographic printing plate according to the electrophotographic process does not require the fountain solution and in addition the plate making can be easily done without etching treatment. Therefore this method might be quite epoch-making if good printed matters could be obtained without causing any troubles. In fact, however, there are various difficulties arising from the use thereof. One of the major problems is that the ink-repellent materials repel not only ink, but also other materials such that they repel even the toner to be deposited and to form image areas, resulting in lowered bonding strength, dropout of the toner in printing and remarkable reduction in the printing durability of the printing plate. Therefore, attempts have been made to modify silicone or fluorocarbon polymer constituting the ink-repellent coating layer in order to improve adhesion between the plate and the toner. Among them, a typical attempt is to introduce a polar group affinitive with the toner into the ink-repellent layer, as disclosed in, for example, Japanese Pat. Laid-Open Nos. 1803/1975, 29305/1977 and 76104/1977, and U.S. Patent Nos. 4,020,761 and 4,259,905.
The formation of a toner image on the dry planographic printing plate according to the electrophotographic process is currently carried out by using conventional toners and no special toners therefor are used. Namely, it is not too much to say that studies on a toner suited for dry planographic printing process have scarcely been made, though attempts to find out a suitable printing plate therefor have been made. Conventional toners are formulated so as to be able to adhere to ordinary cellulose-based paper or coated papers (including conventional plate materials) having a relatively hydrophilic surface so that it cannot be thought that they have a similar affinity with an ink-repellent or hydrophobic surface. Many of conventional toners are epoxy, alkyd or styrene resins, or modified resins thereof. These toners have disadvantages in that they have poor adhesion to a dry planographic printing plate and in some case even several tens of copies can not be printed.